Global surface temperatures are predicted to increase between 1.1° C. to 6.4° C. during the twenty-first century primarily due to increased levels of greenhouse gases (GHGs) in the atmosphere (Solomon et al., 2007). Methane (CH4) is a particularly potent GHG, having a global warming potential 21 times that of carbon dioxide (CO2) (IPCC, 2007) and accounts for 16% of total global GHG emissions (Scheehle & Kruger, 2006). Methane from agriculture represents around 40% of the emissions produced by human-related activities, the single largest source of which is from enteric fermentation in livestock, mainly from ruminant animals (Steinfeld et al., 2006). The worldwide demand for meat and milk is predicted to double by 2050 (Food and Agriculture Organization of the United Nations (FAO), 2008) and ruminant-based agricultural activities are expected to continue to be an important contributor to global CH4 emissions. Therefore, reducing CH4 emissions from ruminants will be important in meeting international commitments under the Kyoto Protocol and also in ensuring the long-term sustainability of ruminant-based agriculture. Moreover, as CH4 production in the rumen accounts for 2-12% of the ingested energy (Johnson & Johnson, 1995), it is predicted that reducing CH4 emissions from ruminants will also make more energy available to the animal and therefore enhance their productivity. Ruminant animals are particularly important to agriculture in New Zealand (NZ), producing a third of NZ's commodity exports (Statistics New Zealand, 2008) and making up a large proportion of the internationally traded lamb and milk products (Leslie et al., 2008). Consequently, NZ has an unusual GHG emission profile, with ruminant CH4 emissions accounting for 31% of NZ's total GHG emissions (Ministry for the Environment, 2007).
Methane is formed in the fore-stomach (reticulorumen, or more commonly known as the rumen) by methanogens, a subgroup of the Archaea. During normal rumen function, plant material is broken down by fibre-degrading microorganisms and fermented mainly to volatile fatty acids (VFAs), ammonia, hydrogen (H2) and CO2. Rumen methanogens principally use H2 to reduce CO2 to CH4 in a series of reactions that are coupled to ATP synthesis. The rumen harbours a variety of different methanogen species, but analyses of archaeal small subunit ribosomal RNA genes from rumen samples of ruminants on differing diets around the world suggest the majority fall into three main groups: Methanobrevibacter, Methanomicrobium and a large, as yet uncultured, group of rumen archaea referred to as rumen cluster C (Janssen & Kirs, 2008). Sequences affiliated with Methanobrevibacter dominate, on average accounting for 61.6% of rumen archaea, with sequences associated with M. gottschalkii (33.6%) and M. ruminantium (27.3%) being most prominent.
Attempts have been made to inhibit the action of methanogens in the rumen using a variety of interventions but most have failed, or met with only limited success, due to low efficacy, poor selectivity, toxicity of compounds against the host, or build up of resistance to anti-methanogen compounds (McAllister & Newbold, 2008). Currently there are few practical methane reduction technologies available for housed ruminant animals, and no effective technologies for grazing animals. Methane interventions should ideally target features that are conserved across all rumen methanogens, so that no unaffected methanogens can fill the vacated niche. Interventions should also be specific for methanogens only, such that other rumen microbes continue their normal digestive functions. Whole genome sequencing allows the definition of gene targets that are both conserved and specific to rumen methanogens. It is not yet possible to obtain genome sequences of all methanogen groups present in the rumen as some are yet to be cultivated, and a rumen methanogen “metagenome” is prevented by the inability of current sequencing technologies to reassemble complete genomes from complex microbial ecosystems. Therefore, sequencing the genomes of individual rumen methanogens currently in culture is a critical step in developing CH4 mitigation technologies for ruminant animals.